McCain is reviewing its £15 million creative account, marking the end of a three-year relationship with TBWA\London.
Beattie McGuinness Bungay and JWT are the front-runners for the account.
TBWA is not repitching and is splitting with the client "by mutual agreement".
Both BMB and JWT have historical ties with the food manufacturer. Trevor Beattie, TBWA's former chairman and executive creative director, was instrumental in winning the account for TBWA, while Alison Burns, the newly appointed chief executive of JWT, has worked on a strategic consultancy basis for the company in the past.
BMB is contractually barred from talking with McCain until May, but JWT has held at least two meetings with McCain in recent weeks.
Sources close to the business suggest that McCain may split the account.
A McCain spokesman said: "We have not made any decision concerning our future advertising agency arrangements. We will review our options over the coming weeks."
A TBWA spokesman added: "McCain's advertising needs have changed. We no longer believe our continued partnership is appropriate."
No intermediary will be involved. McCain's media planning and buying, handled by PHD, remains unaffected.
TBWA won the McCain account in May 2003 following a pitch against Grey London and Euro RSCG Wnek Gosper. The business was previously handled by D'Arcy, but reviewed after D'Arcy's merger into Leo Burnett, where it clashed with Heinz.
The launch of BMB, the TBWA breakaway start-up, last May sparked widespread speculation that the McCain account would defect to it.
In October 2004, TBWA produced a three-and-a-half minute commercial for McCain that celebrated 25 years of its Oven Chips and starred Valerie Singleton. The ad was created by Beattie and directed by Duncan Jones, who has since joined Beattie at BMB.
McCain's current campaign supports its Home Fries and Wedges ranges, and uses the endlines "Taste the chip from heaven" and "Fancy doing something different with potatoes? Leave it to the experts."
In December 2005, McCain appointed Carlson Marketing to launch its first customer loyalty programme. In May of the same year, the company announced its plans to launch a range of delicatessen-style hot sandwiches, McCain Deli Select, backed by part of its £15 million marketing budget.